The present invention is related to a logarithm conversion circuit and the like, which detect light reception input intensity from a current of a photodiode of a light receiving circuit employed in an optical communication system, and monitor the detected light reception input intensity, and more specifically, is related to a temperature compensating circuit thereof.
In general, as is well known in the art, in order to perform a logarithm conversion, both a current under measurement and a reference current are supplied to two sets of bipolar elements, and then, a voltage difference between bases and emitters is derived. However, since a bipolar element contains a coefficient which is directly proportional to an absolute temperature as an inherent characteristic, a gain which is inversely proportional to the absolute temperature must be applied by a temperature compensating circuit in order to obtain a logarithm of a ratio of a current to be measured to a reference current irrespective of a temperature.
Conventionally, a temperature compensating circuit contained in a logarithm conversion circuit is arranged as shown in FIG. 2, as represented in a publication 1 (xe2x80x9cPractical Operational Amplifier Circuitxe2x80x9d written by Hideo Tsunoda, pages 40 to 41, Tokyo Electric University publishing section, Jul. 20, 1983), and a publication 2 (xe2x80x9coperational Amplifierxe2x80x9d edited/translated by Y. Kato, pages 298 to 305, Macgrow Hill K. K., Jun. 30, 1983). Reference numeral 11 shows a temperature sensing resistor, reference numeral 12 indicates a resistor, reference numeral 13 represents an operational amplifier, reference numeral 14 denotes an input terminal, and reference numeral 15 represents an output terminal. This operation is the positive-phase amplifier well known in the art. A voltage gain defined from the input terminal 14 of the positive-phase amplifier to the output terminal 15 thereof may be determined by both the temperature sensing resistor 11 and the resistor 12 of the feedback circuit which is connected to the inverting input terminal of the operational amplifier 13. That is, this voltage gain is equal to (R11+R12)/R11 (note that symbol xe2x80x9cR11xe2x80x9d shows resistance value of temperature sensing resistor 11, and symbol xe2x80x9cR12xe2x80x9d indicates resistance value of resistor 12). Since the temperature sensing resistor 11 owns such a temperature coefficient of approximately 0.39%/xc2x0 C., such a gain is obtained which is inversely proportional to the absolute temperature. As the temperature sensing resistor 11, a metal such as platinum is utilized as a resistance member.
An object of the present invention is to provide a low-cost light receiving monitor circuit operable in high precision without a temperature dependent characteristic, while employing a logarithm conversion circuit containing a temperature compensating circuit, is also to provide a light receiver and an optical communication system.
Very recently, in an optical communication system, since light reception input intensity is monitored, abnormal conditions of a light transmitter, a transmission path, and the like, which constitute light transmission sides, are detected. Then, warning information of the abnormal conditions, and also interruption of the optical communication system are required.
However, when a current derived from a photodiode is merely monitored, for instance, in accordance with the recommendations of the ITU-T (International Telecommunication Union-Telecommunication Standardization Section), the light reception input strength becomes xe2x88x9228 to xe2x88x928 dBm in STM-4, and becomes xe2x88x9228 to xe2x88x929 dBm in SMT-16. It is required to represent such a light reception input strength range which is approximately 100 times higher than the light reception input strength. As a result, a logarithm conversion circuit for converting the light reception input strength into a strength in a logarithm scale is necessarily required.
Furthermore, there are many cases that an avalanche photodiode (will be abbreviated as an xe2x80x9cAPDxe2x80x9d hereinafter) is employed, while optical communication systems are constructed in long transmission paths. In general, an APD is used in such a manner that a current multiplication ratio specific to this APD is reduced in a high level range of a light reception input level. As a result, in order to monitor a current of the avalanche photodiode, it is desirable to provide a function for correcting a change in the current multiplication ratio other than a temperature compensation made in a logarithm conversion circuit.
Also, since the high-cost platinum resistors and the like are used in the conventional temperature compensating circuit, there is a drawback that the low-cost light receiving monitor circuit could not be realized.
The present invention may remove the above-described drawbacks, and may provide a low-cost light receiving monitor circuit operable in high precision without having a temperature dependent characteristic, while employing a logarithm conversion circuit containing a temperature compensating circuit, and also may provide a light receiver and an optical communication system.
A temperature compensating circuit, according to an aspect of the present invention, is featured by comprising: a first circuit network between an inverting input terminal of an operational amplifier and an output terminal of the operational amplifier; and a second circuit network between the inverting input terminal of the operational amplifier and a reference potential; wherein: at least one of the first circuit network and the second circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and the temperature compensating circuit compensates a temperature-dependent signal which is inputted into a positive phase input terminal of the operational amplifier, and outputs the temperature-compensated signal.
Also, a temperature compensating circuit, according to a modification of the present invention, is featured by comprising: a first circuit network between an inverting input terminal of an operational amplifier and an output terminal of the operational amplifier; and a second circuit network between the inverting input terminal of the operational amplifier and a signal input terminal thereof; wherein: a positive phase input terminal of the operational amplifier is connected to the ground; at least one of the first circuit network and the second circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and the temperature compensating circuit compensates a temperature-dependent signal which is inputted into the signal input terminal of the operational amplifier, and outputs the temperature-compensated signal.
Also, the temperature compensating circuit is featured by comprising a logarithm converting circuit, wherein: an output terminal of the logarithm converting circuit is connected to the input of the above-explained temperature compensating circuit.
Also, a temperature compensating logarithm circuit, according to another aspect of the present invention, is featured by comprising: a third circuit network between the output terminal of the above-explained temperature compensating logarithm converting circuit and a positive phase input terminal of a second operational amplifier; and a fourth circuit network between the output terminal of the second operational amplifier and the positive phase input terminal thereof; in which at least one of the third circuit network and the fourth circuit network is made of an arrangement containing a plurality of series-connected thermistor/resistor pairs in which the thermistors are connected parallel to the resistors; and in the case that a gain defined from the inverting input terminal of the second temperature compensating circuit up to the output of the second operational amplifier is equal to (1+G), a gain defined from the positive phase input terminal of the pre-staged operational amplifier up to the output of the pre-staged operational amplifier is nearly equal to (1+(1/G)).
Also, a temperature compensating logarithm converting circuit, according to another aspect of the present invention, is featured by comprising: an adding circuit for entering thereinto a voltage of one output unit of the logarithm converting circuit as a first input, and for entering thereinto a voltage of the other output unit of the logarithm converting circuit as a second input, in which an output of the adding circuit is equal to a summation between p-times multiplied first input voltage (p greater than 0) and (1xe2x88x92p) times multiplied second input voltage; a bipolar element in which the output voltage of the adding circuit is applied to the emitter thereof; a resistor whose one terminal is connected to the base of the bipolar element; and a voltage follower circuit in which one terminal of the resistor is connected to the input unit thereof, and the output unit thereof is connected to the first voltage; wherein: a potential difference between the output unit of the second operational amplifier and the other terminal of the resistor is outputted.
Also, the temperature compensating logarithm converting circuit is featured by that the second input of the adding circuit is a voltage appeared on the second output terminal of the logarithm converting circuit.
Also, the temperature compensating logarithm converting circuit is featured by that the adding circuit is comprised of a resistor divider, and a second voltage follower for entering thereinto a potential at an intermediate point of the resistor divider; both ends of the resistor divider are the input terminals of the adding circuit; and the output terminals of the second voltage follower are the output terminals of the adding circuit.
Also, a light receiver, according to another aspect of the present invention, is featured by comprising: a light receiving element for receiving a light signal; a first amplifier for amplifying the signal which is photo-electrically converted by the light receiving element; a second amplifier for further amplifying the output signal of the first amplifier; a discriminating/reproducing device for discriminating/reproducing the output of the second amplifier to output the reproduced signal; a timing extracting device for extracting a clock component from the output of the second amplifier to output the clock; and a temperature compensating logarithm converting circuit for entering thereinto a portion of the output signal of the light receiving element to convert the entered signal into a logarithm, whereby a light reception input intensity monitored signal is outputted.
As previously described in detail, in accordance with the present invention, while using the logarithm converting circuit containing the temperature compensating circuit, it is possible to provide a low-cost light reception monitor circuit, a low-cost light receiver, and a low-cost optical communication system, which are operable in high precision without having the temperature dependent characteristic.